Apparatus for measuring electric resistances.



110.812,19. PATENTED FEB. 13, 1906. S. EVERSHED.

APPARATUS FOR MEASURING ELECTRIC RESISTANCES.

APPLICATION FILED PEB.27.1904.

5 SHEETS-SHEET 1.

n y I PATENTED FEB. 13, 1906.

S. EVERSHBD.

PPARATUS FOR MBASURING ELECTRIC RESISTANGBS.

APPLICATION FILED P21127. 1904. 5 sHEBTs-sHBBT 2.

No. 812,196. PATENTED FEB. 13, 1906. S. EVERSHED.

APPARATUS FOR MEASURING ELEGTRIG RESISTANCESI APPLICATION FILBD FBB.27,1904.

5 SHEETS-SHEET 3.

PATENTED EEB. 13, 1906.

S. EVERSHED.

APPARATUS ECR MEASURING ELECTRIC RESISTANCES.

APPLICATION FILED FEB.27,1904.

5 SHEETSSHEET 4.

F1 GA.

4 ,M 5 .1' T. www

No. 812,196. PATENTED FEB.13, 1906. s. lBVERSHBD. APPARATUS FUR MBASURING ELECTRIC RBssTANCES.

AFPLXGATION FILED PEB.27,1904.

5 SHEETSFSHEBT 5.

U'Nlrrnn srA'rns PATENT carica.

SYDNEY EVERSHED, STREATLEYENGIAND, ASSIGNOR TO H'IMSELE, AND EVERSHED AND VIGNOLES LIMITED. OF CHISWICK, ENGLAND,

A CORPORATION'.

Specification of Letters Patent.

Patented Feb. 13, 1906.

Application filed February 27, 1904. Serial No. 195,610.

To aZZ whom t may concern:

Be it known that I, SYDNEY EvERsr-IED, a

\ subject of the King of Great Britain and 1reland,fresiding at Streatley, in the county of Berks, En land, have invented a certain new and usefu Improvement' in Apparatus for Measuring Electric Resistances, of which the following is a specification.

This invention relates to apparatus used for measuring electricresistance, and particularly insulation resistance, and it is specially applicable as a portable testing set intended for testing and measuring the insulation of electric circuits at voltages commensurate with the Working voltage of the circuits. Such testing sets as heretofore known generally comprise a portable hand-dynamo 'or other convenient source of electric energy anda direct-reading ohmmeter or equivalent instrument for the measurement of resistance. The ohmmeter in such known sets has been of the ordinary type in which fixed coils act upon a moving magnet. This ty e of instrument has several disadvantages or this particular purpose. 4 The magnetic fields of the coils are necessarily feeble when very high resistances are under test, so that even when the magnet system is made nearly astatic by any of the Well-known methods stray magnetic forces are apt'to produce errors inthe indications of the ohmmeter.l Hence Vii a hand-dynamo is used as the source of current it must be placed at some little distance from the ohmmeter while in use. For this reason the ohmmeter and dynamo have been heretofore fitted in separate boxes or cases, and before they can be brought into use 1t 1s neces--` sary to electrically connect the two. The operative forces upon the needle or magnet system of the ohmmeter are feeble even when the coils are formed of an enormous number of turns of the finest procurable wire, so that very delicate pivoting ofthe moving system is essential to avoid errors due to friction. Testing Asets ofthe typereferred to have not been available for testing insulation resistances above about one hundred' megohms,`

because the limit of sensitiveness in the ohmmeters heretofore used has been reached by Winding the coils with the finest procurabie wire, and it has been impracticable to further increase the operative forces.

The objects of my invention are generally to increase the portability, scope of utility, and convenience of apparatus ofthe kind specified by the use of ohmmeters or equivalent instruments based upon principles which render their working and indication entirely independent of stray magnetic fields, so that I can not only use my ohmmeter in quite close proximity to the hand-dynamo, but

even hx the ohmmeter and dynamo in one box or case, thus greatly increasing the portabilitivv and convenience of the apparatus.

A type of instrument entirely unaffected by stray fields is available for the purposes of thisinvention-namely, the moving-coils t e.

yrPlhis instrument is well known in its application for other purposes; but in ap lying it for the purpose of this invention 1P modify and adapt it with special reference to lthe particular and novel functions it is intended to perform in ways hereinafter described.

Itv is an essential feature of testing sets of the type hereinafter described that the index of the indicating instrument shall'range over a scalein ohms or megohms, and it is to be understood that I make it direct reading in this sense. l

According to this invention a portable testing apparatus well adapted for carrying out tests at moderate pressures is constructed as follows:

Figure-ishows a preferable arran ement of coils, pole-pieces, and magnetic shield for. `the moving-coil ohmmeter, detached and on a large scale. Fig. 1a is a diagram of the electric connections of this ap aratus. Fig. 2 is an elevation of the coils o the said ohmmeter and illustrates a referable `form of iiexible connections with device for releasing torsion, device. of connection by s ring-brushes with releasingear. Fig. 5 s ows-'another alternative Fig. 3 is a plan ofthe said releasin i Fig. 4 shows an alternative metho met od inwhich the flexible conductors are collsA and B, fixed to an axle carrying a pointer and adaptedto rotate in a fixed magnetic field. The angular position of the movable IOO IIo

coils being uncontrolled by an mechanical means, the position they will ta e up will depend solely upon their relative turning moments. The couple tending to deflect the current-coil A will be proportional to the strength of current traversing 'the unknown resistance Z', `while the restoring lcouple of the pressure-coil B will be pro ortional to the current flowing in the circuit B k l and to a function of the angle'the coil makes with the field due to the two poles N S.

When theunknown resistance is, infinite,

the current in the current-coil being zero, the

' k2 7c3 are short-circuited. The position so' pressure-coil alone determines the angular osition of the movable system and natura ly the pressurecoil moves round until it lies with'its plane at right angles to the magnetic eld, in which position it is shown in the diaam, Fig. l, This position is marked Innity on the dial.

When the unknown resistance is reduced to zero, (short circuit across the terminals lf2 c'3,)`

the current in the current-coil will be a 'maximum and will tend to drive it right out of the magnetic field. The resistance k is inserted to prevent this and to insure a position of stable equilibrium of the movable system when taken up is marked Zero on the dialyb'utit is to be understood that the apparatus is 1n reality measuring the resistance le', By adjusting the value of k the zero-point may be rolught to anyy required position upon the Vit Win be noticed that while both sidtsbf the current-coil are operative, each being o n uniform to one where it lies within the one Aof the airaps of the magnet system, the pressure-coi has only one operative side, the other s ide being placed at the center of the hollow iron core wherev no 4forctrcan act upon it. This is rendered necessary for conystructional reasons, as otherwise the second side of the pressure-coil would foul the bracket which serves to support the core. The arrangement and dis osition of `thepressurecoil is well adapte for the pur oses of an ohmmeter, since the coil moves om a position where the field is very weak and nearly owerful ield within one ofthe air-gaps. ence the ohmmeter serves to measure a'wide range of-resistance upon a single scale Without the use of shunts or multiplyingdevices.

Like letters refer to similar parts and partsy core for that 'magnetic field, and or the 'invention so arrange the po e-pieces rela-l pecially to great sensibility. In my preferable arrangement the coil which is in serieswith the resistance under test,hereinafter referred to as the current-coil, moves in the uniform radial magnetic field of a fixedinagnet,which is fitted with two poleieces and a central ur o se, t ecurrent-coil being free to rotate'in t e narrow annular air-gaps between the pole-pieces and the core, as mthe Well-known moving-coil Voltmeters and ammeters. shunt-circuit of suitable resistance, hereinafter called the pressure-coi is fixed upon lthe 'same axis as and at al convenient an le with the current-coil. The currents in 't e two coils How in opposite directions, so as to produce opposingforces upon the coils. When the resistance under test is infinite, thecurrent-'coil carries no current, and hence the pressure-coil alone determines the angular position of the twocoils. Under these circumstancesthe systemucomes to rest with the magnetic axis ,of the'pressure-coil in line with' etic axis of the xed lo'rparallel to the ma urpose of my tively to the coils that the ressure-coil lies in a Weak'eld outside the alr-gaps, and hence coil a considerable deflection will take place from infinity. Conversely, at zero the'pr'essure-coil will 'be in a magnetic fieldof maxi- A second coil in series with a' vif avery smallcurrent-fiows in the currentmumstrength.- The current-coil,on the otherv hand, moves in the'uniformiield of the annular gaps.v Theseare essential features oi' my improved ohmmeter to give the required sensitiveness. c

'When'ne'cessaryn order toobtain greater IOO sensitiveness at or nearthe infinity-mark 1 upon the scale, I still further reduce the strengthof the field acting upon the pressurecoil at this position by fixing an iron plate to the framework of the instrument in such a position as to screen the pressure-coil,

' `In knownmoving-coil instruments the cur-f rent is led to and from the coils by means of -fiexible conductorsorrby the springs which act as the controlling force. In amovingcoil instrument, used as an ohmmeter,y there ought to be no control u on the moving system other than that of t e coils themselves,

and the effect of even the slighttorsionof the flexible conductors is to cause an error in the readings of the instrument. In applying a moving-coil instrument, as an ohmmeter, I therefore provide a novel form of duplex flexible conductors so disposed as to permit 'of their torsion bein entirely removed without `at any time brea ing the circuits of the instrument'. tors, (each in duplex form.) One serves to I general y provide three conduccarry current to the common terminal of the two coils, and the other two serve to lead the separate currents away againfrom the pressure-coil and current-coil. Each of these iro ISO

three flexible conductors bein double, I provide means for temporarily li erating them, and so releasing the elastic tor ue or torsion of one conductor out of each'of t e three pairs. The torsion being thus removed from them, they are once more clamped to the fixed terminals from which they proceed, and the other three conductors are similarly released, and their torsion removed. By successive releasings and clampings of the conductors the whole of the torsion due to the three dupl ex conductors is finally removed, and hence the coils and index finally come to the true position corresponding with the resistance under test. The alternate releasing and clamping of the flexible conductors is preferably effected by mechanical means operated by hand or otherwise; but I may alternatively use the armature of cleetromagnets for the same purpose and effect the alternate releasing and clamping by switching the magnets on and oli, or I may use steel hairsprings as my duplex flexible conductors and cause the clamping of the springs to take place by the direct attraction of the electromagnets, whose poles will in this modification form the fixed terminals of the moving-coil system. In another modification by which torsionless connections may be made between the moving coils and their fixed terminals the terminal wires of the coils are connected to three slip-rings upon the axle of the movable system, and six elastic collecting-brushes are arranged in contact with the rings, two brushes bearing upon each ring. Means are then provided or alternating, lifting, and replaeingthe brushes three at a time in an analogous manner to the releasing and clamping of the flexible conduct-ors already described. In this way both the friction-torque and the elastic torque of the brushes may be entirely eliminated.

My testing apparatus is completed by a suitable hand-dynamo designed to furnish continuous currents at the requiredvoltage. The machine described in the specification of United States Patent No. 639,849 is well adapted for the purpose. The ohmmeter and dynamo are both fixed inside the same box in order to reduce the weight. of the whole as much as possible. VVih the same object I make one field-magnet serve both to excite the dynamo and to provide the magnetic field of the ohmmeter, an arrangement which leads to a further 'considerable reduction in weight. The field-magnet may be either a permanent magnet or an electromagnet. The combination of my imnroved moving-coil ohmmeter with a dynamo in one box enables me to provide for operating my torsion-releasing mechanism by a cam, crank, or equivalent device upon a suitable spindle geared to the dynamo-spindle, so that the necessary releasing and clamping takes place automatically Whenever the dynamo is driven. A further obvious advantage inherent in my combined ohnuneter and dynamo is that the electrical connection between them is made once for all by wires inside the box, and the apparatus is therefore ready for use at any moment without preparation.

In Fig. 1, A is the current-coil, preferabl wound on a copper bobbin to damp the osci lations of the instrument. It is mounted on an axle a, with indicator-pointer (LX attached.r B is the pressure-coil, which has only one side operative, the other passing down a hole in the center of the iron core C. The currentcoil moves in the annular air-gaps between the pole-pieces N S and the core C, while the operative side of the pressure-coil moves from a position midway between the horns ofthe two pole-pieces to a position within the annular airgap. The necessary magnetic induction is provided by means of magnets D D, which may either be permanent magnets or electromagnets. The coils are shown in the position they occupy when the resistance to be measured being infinite there is no current through the current-coil A, and the system is therefore directed solely b the force upon the 'pressure-coil B. Io o tain sufficient sensitiveness to measure very hi h resistances, it is necessary to so arrange tne pressure-coil with regard to the magnetic field that it may have a very feeble controlling force in the infinity position and a maximum force when the resistance is a minimum. This is secured, according to myinvention, by the particular'arrangement shown in F ig. 1, and to further increase the sensibility I provide an iron plate b, which diminishes the strength of the field in the neighborhood of the operative side of the pressure-coil, more or less, according to the position and dimensions of the plate. `Means are provided for adjusting the position of the plate b in order to secure therequired sensibility.

In Fig. 2 the pressure-coil B and currentcoil A are seen in elevation, the pole-'pieces N S and core C being shown in section. One end of the winding of each coil is soldered to the axle c of the current-coil, which forms a common terminal of the coils. The other end of the current-coil A is connected to an insulated sleeve a upon the axle, to which two flexible conductors or springs E2 and E5 are soldered. Similarly the other end of the pressure-coil B is led to an insulated sleeve a2, to which the springs E3 and E are fixed. A third sleeve a3, which is not insulated, but

"in direct connection with the axle, carries two more flexible conductors E. and E". The free ends of the flexible conductors are made to project outward radially, three on one side of the axle and three on the o posite side. I find it advantageous to provi e some elasticity in the free ends of the springs, which may be conveniently effected by doubling the end back, as shown iii Fig. 2. Six

IOO

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spring vises or clamps e', e2, e3, e4, e5, and e are so fixed that the six free ends of the springs can be clam ed by them. In the drawin s the sprin s E', E2, and E3 are shown tight y 'clamped y their respective vises, While the jaws of the three vises e, e5, and e being open the corresponding springs E4, E5, and E6 are entirelyfree from restraint. The alternate clamping and releasing of the flexible conductors are performed by means of a parallel link device' F, which oscill'ates about two centers f and f. The link device carries three insulated rollers f2, which act as tappets to depress the movable jaws of the vises e', e2, and e3. Three similar rollers f3 perform a like function for the vises e4, e5, and e. The oscillatory motion is imparted to the link device by means of a spindle G, which carries thecrank g and connecting-rod g. The arrangement of the electrical circuits is described later in reference to Fig. 6.

\ In Fig. 3 the arrangement of the releasing device is shown in' plan. In this drawing only two of the vises e and e* and flexible connectors Ef and E* are shown in order to avoid confusion. The spindle G can be driven 1n any 'convenlent manner either by hand or otherwise.

In Fig. 3 I show in dotted lines a smallf meter I provide insulated slip-rings E7, &c., which form the terminals of the coils. One of these, E7, is shown'in Fig. 4, and connection with it isalternatel made by two springbrushes e7 .and el". T ese brushesare attached tov levers H" and H4, pivoted on centers h and h4. The necessary oscillating motion is imparted to the levers by means of a roller gf, pivoted on an eccentric-pin g5 on any revolving axle G. The brushes e7 and ew are l pressed into contact with the slip-ring by' means of a suitable s ring k7, connecting the levers H and H4, an the pressure applied to the sli -ring is limited by means of the stop'hs.

In ig. -5 is shown another formv of-my releasing device in which the flexible conduc-.

tors E14 to E19 are alternately clamped and released by the magnetic attraction of electromagnets. In this modification the flexible conductors E14 to E19 may conveniently consist of thin strai ht iron wires arranged parallel with the o meter-axle and con- Vcentrieally disposed around it. Their free In this modification instead of fixing flexible conductors to the axle of the ohm-- ends lie Within the circular space formed by the pole-pieces of six electromagnets J to Je (Better seen in lan, Fig. l6.) These magnets are mounte u on al metallic plate j7, the magnetic circuits o the magnets y2, ja, 7'5, and j being insulated from the plate i7, while those of the magnets y" andj* are electrically connected to j?. The coils of these electromagnets are coupled to two circuits and y,

the three coils j', jg, and j? being in connection with circuit y and the other three coils i4, js, and j in connectionwith the circuit The magnets J3 and J erform the same functions as the vises e2 an e5 in Fig. 2, and the are therefore electrically connected toget er. The same remarkA refers to J2 and J5, which correspond to the vises e3 and e in Fig. 2, while J and J4 correspond to the vises e and e4 in Fig. 2. 'The currents in the two circuits and two sets of coils are controlled by means of a rotary switch which serves to excite first the magnets J J2, and J? and then the magnets J, J5, and J", th'us alternately attracting and fixing three of the flexible conductors and then attracting the other three and liberating the first three. The magnets form the fixed terminals of the ohmmetercoils, and in order to insure good electrical contact between the flexible conductors and the pole-pieces of the magnets the Wires and the poleieces may be gilt.

Instea lof iron wires I lmay useany other suitable metal, whether magnetic or not, and attach small pieces of iron to their free ends to serve as armatures to the electromagnets.

The electrical connections of this movingcoil ohmmeter are illustrated in Fi 1, Fig. 2, and again in Fig. 6, and the funct1on of the apparatus is common to the different constructions shown. Current is derived from the dynamo K, a resistance c of ap ropriate value being inserted in the circuit o the pres,- sure-coil, while in series with the current-coil is a resistance c, which serves to adjust the zero position of the index of the instrument to the required point on the dial; An unknown res1stance which is to be measured or tested is connected between the,l terminals k2 and k3. It is to be understood that the vises f shown in Figs. 2 and 3 and the corres onding parts in Figs. 4, 5, and 6 are fixed y insuating-supports to a suitable framework, consisting of a lower plate R, anup er plate 1', and corner angle-pillars, which is sliown in Fig. 7,.

In order to prevent leakage across the insulators in the ohmmeter from iniiuencingthe lator of the current-coil circuit inthe instru- IOO IIO

ment on a guard-plate so connected. This has the effect of preventing any large potential difference across the msulating-pieces, and therefore eliminating leakage. The princilple of the arrangement is Well seen in Fig. 7, w 1ere the insulator to which k2 is attached and the resistance le are mounted upon guardplates k, which are in the drawings in electrical connection with the common terminal of the ohmmeter-coils, so that this guard- Elate performs the same function as the wellnown Prices guard-wire used in testing the insulation of cables. It is only necessary to provide guard-plates for the various insulating-pieces which support conductors or terminals in direct connection with the currentcoil and its circuit.

In Fig. 7 I show a preferable arrangement of bar-magnets D D, which provide for the induction in the dynamo K and for the field in the ohmmeter. I show permanent magnets; but I may use electromagnets, in which case the exciting-coils may conveniently form part of the resistance c in series With the pressure-coil. The terminals k2 and k3, to which the circuits or cables to be tested are to be attached, are conveniently placed on one side of the containing box or case Z, and

the dial N and index a are observed through a glazed opening in the top` of the box. A scale of ohms or megohms is marked upon .the dial, so that the instrument is direct-readmg.

Having now described my invention, what I claim, and desire to secure by Letters Patent, is-

1. In combination, a generator of electric current, in close proximity to said generator, a direct-reading movin -coils ohmmeter with two coils, and a orta le box inclosing vthe same for testing e ectrical resistance.

2. In combination, a' generator of electric current; a direct-reading moving-coils ohmmeter in close proximity thereto; means for releasing the torsion of the conductors which convey current to and from the moving coils; and a ortable box inclosing the same.

3. n-combination, a generator of an electrical current, a direct-reading moving-coils ohmmeter in close proximit thereto; means for releasing the torsion o the conductors which conveycurrent to and from the moving coils; electrically-guarded insulations to avoid leakage fand a portable box inclosing the same.

4. In combination, a hand-operated dynamo a direct-reading moving-coils ohmmeter in close proximity thereto; du lex flexible conductors to the moving colIs of said ohmmeter; fixed clamps adapted to alternately clamp and release the said conductors;

means for actuating the clamps; and a' portable box inclosing the same.

5. In combination, a direct-readin ohmmeter having moving coils and an in ex-finger rotatable relatively to fixed magnetic poles; flexible conductors to the moving coils eakage and a portable box inclosing said apparatus.

7. In combination, a generator of electric current; a direct-reading ohmmeter having moving coils and an index fin errotatable relatively to fixed magnetic po es said coils being connected at one end to the axis of said finger; a air of flexible conductors connected to the ot er end of one of the movable coils; a pair of flexible conductors insulated from the said axis and connected to the other end of the other movable coil; six contacts about the said axis adapted to connect said flexible conductors to the line-circuits; and means 1 for closing and releasing the said contacts Withy the said flexible conductors intermit-` tently. 4 l

8. In combination, a hand-operated dynamo; a direct -reading ohmmeter having moving coils and an index-fin er rotatable relatively to a xed magnet; sail coils having their common terminal connected With the axis of the said finger; a pair of flexible conductors connected to the said axis; a pair of flexible conductors upon an insulated sleeve of the said axis, and connected to the other end of one of the movable coils; a pair of flexible conductors upon an insulated sleeve of the said axis and connected to the other end of the Vother movable coil; six contact- IOC clamps arranged about the saidv axis and ada ted to' make contact with said flexible con uctors; circuits thereto from dynamo and external terminals; and means, o erated from the dynamo, for causing the sai clampcontacts to intermittently and alternate make contact with, and release, the said flexible conductors.

In witness whereof I have hereunto set my hand in`presence of' two witnesses.

SYDNEY EvnRsH-ED.

Witnesses:

JOHN C. FELL, CHARLES CARTER IIO 

